Trees, shrubs, subshrubs, or somewhat vinelike, solitary to forming mats or clumps, terrestrial (sometimes deep-seated in substrate) to epiphytic or epipetric, erect to sprawling (rarely scrambling or climbing) or pendent in epiphytic or epipetric taxa, simple to many branched, usually stem succulent. Roots diffuse, taproots, or tuberlike, sometimes adventitious. Stems unsegmented or segmented, segments persistent to easily detachable; long shoots spheric to depressed-spheric or club-shaped to long cylindric, or sometimes flattened cladodes, smooth, tuberculate and/or fluted with ribs; tubercles distinct as nipple-shaped or ridgelike (to triangular or pyramidal) protuberances to coalescent as vertical ribs; ribs 2-30[-40+], if ribs 2, stems winged, if ribs 3 or more, stems ± angled; short shoots (areoles) positioned on crests of ribs, at or near tubercle apices, or in axils of tubercles, commonly bearing persistent spines, also minute, barbed, deciduous spines (glochids) in subfam. Opuntioideae, and abundant, dense hairs (wool) creating a cushionlike appearance. Leaves deciduous to persistent, vestigial or absent, spirally alternate, sessile (petiolate to subsessile in Pereskia and several genera outside the flora), terete or flat, 0-3 cm (to 10 cm in Pereskia); stipules absent. Spines flexible and hairlike or bristlelike to rigid and needlelike or nail-like, terete to angled or flat, mostly hard (rarely corky or papery). Flowers bisexual (rarely unisexual or with bisexual and pistillate flowers on separate plants), nocturnal or diurnal, 1(-several) per areole, arranged in true inflorescence only in subfam. Pereskioideae, or chains of fruits proliferating from fruit areoles (in Cylindropuntia fulgida), sessile (pedicellate in Pereskia), arising from stem areole at apex or axil of tubercle, radially symmetric [bilaterally symmetric]; flower tube 0.2-15[-30] cm; perianth epigynous (perigynous in some Pereskia), deciduous or persistent on fruit; tepals 5-50 or more, intergrading gradually from bractlike or sepal-like outer tepals to petal-like inner tepals; stamens usually 50-1500+ [sometimes fewer], decurrent on inner surface of flower tube; true ovary sunken in stem with tubercles present or absent, areoles conspicuous to obscure or absent; subtending scales persistent or deciduous, sometimes absent; spines present or absent, glochids present only in subfam. O The Cactaceae had a New World origin. Only Rhipsalis, a tropical, epiphytic 'mistletoe cactus' with seeds dispersed by birds, has colonized forests of Africa, Madagascar, and Sri Lanka, apparently without human assistance. Other genera of cacti, especially Opuntia, have become naturalized in dry regions of Africa, Eurasia, and Australia, where they have been introduced for food, fodder, ornament, and the formerly very lucrative cochineal dye industry. In some places where they vegetatively clone, cacti have become weedy. A number of taxa throughout the New World have been used by humans in various ways for about 12,000 years (E. F. Anderson 2001). The nomenclature and taxonomic circumscriptions presented here are based on recent, independent studies and differ significantly from the work of L. D. Benson (1982) and older publications. In particular, many taxa previously combined and recognized in the broad sense are now known as biologically distinct taxa, and other taxa appearing as sympatric 'varieties' within species on Benson´s distribution maps have since been resolved either as fully isolated biological species or as misidentified or mismapped records of allopatric geographic taxa.

The family is now divided into four subfamilies (R. E. Wallace 1995; E. F. Anderson 2001). Three subfamilies occur in North America: Pereskioideae, Opuntioideae, and Cactoideae. The fourth subfamily, Maihuenioideae, is restricted to South America (Argentina and Chile).

The Cactaceae maintain an unusually complete representation of their phylogenetic history. Subfamily Pereskioideae, with the least-derived traits, consists of trees, shrubs, or scrambling 'vines' with broad, seasonally deciduous leaves and terete stems that are only weakly succulent. As broad-leaved plants, their relationship to other cacti is not readily apparent, especially if flowers are absent. The succulent trees, shrubs, mat-forming subshrubs, and few geophytes of subfam. Opuntioideae have mostly very short-lived, terete, cylindric, or conic leaves, usually present only on young growth and flowers. Plants of subfam. Cactoideae have vestigial, usually minute (or absent) leaves and extremely succulent, spheric, barrel-shaped to columnar or snakelike stems. Similarly, flowers of Cactaceae range from the simple, radially symmetric, bee-pollinated flowers of most cacti to the elaborate, bilaterally symmetric, hummingbird-pollinated flowers of, e.g., Schlumbergera, the commercially grown 'Christmas cactus' or 'Easter cactus.' Funnelform bat-pollinated flowers also occur on some of the tallest cacti. The family ranges from extreme xerophytes in deserts to epiphytic mesophytes in rain forest, from sea level to about 4500 m (the altiplano of South American Andes), and from the equator to about 56º north latitude (Opuntia fragilis) and 50º latitude in the Southern Hemisphere (E. F. Anderson 2000). Whereas most cacti are terrestrial, in the tropics several genera are epiphytic on trees. A few tropical genera are also epipetric, and a number of xeric taxa may occur exclusively (or nearly so) in rock fissures.